Thermoplastic polymers are commonly used to manufacture various shaped articles that may be used in applications such as automotive parts, food containers, signs, and packaging materials. Shaped articles comprising polyester may be prepared from the molten polymer by a number of melt extrusion processes known in the art, such as injection molding, compression molding, blow molding, and profile extrusion.
Shaped articles may also be produced by thermoforming in which a thermoplastic film or sheet is heated above its softening temperature and formed into a desired shape. This formed sheet of a film or laminate is usually referred to as a forming web. Various systems and devices are used in a thermoforming process, often accompanied by vacuum-assist and plug-assist components to provide the proper forming of the forming web into a predetermined shape. Thermoforming may produce many packaging articles such as cups, trays, and “clam shell” packages.
The most common polyester currently used is polyethylene terephthalate (PET). Due to recent trends toward sustainability and reduced use of petroleum, alternatives to PET are being investigated. Poly(trimethylene terephthalate), herein abbreviated 3GT, also referred to as polypropylene terephthalate or PTT, may be useful in many materials and products in which polyesters such as PET are currently used, for example molded articles. 3GT has properties including a semi-crystalline molecular structure.
British Patent 578097 disclosed the synthesis of 3GT in 1941. 3GT may be prepared using 1,3-propanediol derived from petroleum sources or from biological processes using renewable resources (“bio-based” synthesis). The ability to prepare 3GT from renewable resources makes it an attractive alternative to PET. 3GT produced from renewable sources of 1,3-propanediol is commercially available from E. I. du Pont de Nemours and Company (DuPont) under the tradename SORONA. DuPont pioneered a way to produce the 1,3-propanediol from renewable resources such as corn sugar.
Properties desirable in a thermoplastic composition depend on the article to be produced from the composition and the process used to prepare the article.
For example, a resin, sheet or film suitable for thermoforming process desirably has more than 100% elongation at the thermoforming temperature. It may also be desirable for the resin to have relatively high melt viscosity to improve the rheology for extrusion or coextrusion (in multilayer sheets). Compositions with high crystallization onset temperature from the melt may help ensure that a sheet of the composition may retain dimensional stability during subsequent heating/softening above the glass transition temperature (Tg) prior to vacuum/plug-assist thermoforming.
Alternatively, compositions useful for injection molding applications may require reduced melt viscosity or increased melt flow. Benefits may include better mold filling because of the higher flow, a broader processing window (lower melt temperatures, higher shear regions, and small gate sizes) and articles that have less molded-in stress. In addition, more complete crystallization from the melt of compositions is desirable in molding since an incompletely crystallized part may be more difficult to mold and if heated to elevated temperature may begin to crystallize, leading to shape distortion of the part. Molded-in stresses are relieved as the temperature is raised and crystallization further changes the shape of the molded part.
Toughening (increased impact resistance) may also be useful for articles prepared from the compositions. Toughening polyester has been achieved using an ionomer modifier, an epoxide-containing copolymer such as ethylene/n-butyl acrylate/glycidyl methacrylate (e.g., WO85/03718, WO2007/089644, and U.S. Pat. No. 5,091,478). See also, JP2,614,200, JP2004-300376, and JP2006-290952.
It is desirable to develop 3GT polyester resins that are crystallized and optionally toughened, but with controlled melt viscosity, so that they are suitable to produce a variety of products.